1. Field of the Invention
The present invention relates to a method of fabricating a liquid crystal panel including a liquid crystal/resin composite layer as a light modulation layer.
2. Description of the Prior Art
A display device employing a liquid crystal display panel has been studied and developed widely because of its compact size and its light weight. Recently, pocket television sets employing a twisted nematic (TN) mode liquid crystal display panel have been used practically. Furthermore, image projection type television systems, view finders, and the like using the liquid crystal display panel as a light bulb have also been used practically.
However, a TN mode liquid crystal display panel has disadvantages such as low brightness of display due to the two polarization plates needed for light modulation. Further, fabrication steps are complex. For example, rubbing is needed in order to align liquid crystal molecules.
Recently, liquid crystal panels with use of liquid crystal/resin composite have attracted attention because it need no polarizing plates and no rubbing step on fabrication. A liquid crystal/resin composite is a composite of a liquid crystal component and a resin component, and the liquid crystal component extends in the matrix made of the resin (polymer) component. The structure of the liquid crystal/resin composite changes according to the ratio of the liquid crystal component to the resin component, and it is classified typically into two types according to the ratio of the liquid crystal component to the polymer (resin) component. In one type of the composite having a small ratio of the liquid crystal component, droplets of liquid crystal are dispersed in the voids of the resin layer, and the liquid crystal exists discontinuously, as shown schematically in FIGS. 1(a) and 1(b). This type is called as polymer dispersed liquid crystal. If the amount of the liquid crystal component is increased, droplets contact with each other to form a continuous phase. In the other type having a large ratio of the liquid crystal component, a network of the resin component is formed and the liquid crystal exists not as droplets, but extends in the network continuously. This structure is analogous to a sponge impregnated with liquid crystal. This type is called as polymer network liquid crystal.
The above-mentioned two types of liquid crystal/polymer composite can be used to display an image by controlling the scattering and transmission of light. A display panel with use of the polymer dispersed liquid crystal uses a property that the refractive index of the liquid crystal varies with the orientation direction of the liquid crystal molecules. FIG. 1(a) shows schematically a polymer dispersion liquid crystal layer 136 interposed between two substrates 131 and 132 each having thin film transistors 134 and pixel electrodes 135 and a counterelectrode 133. The polymer dispersion liquid crystal 136 comprises liquid crystal droplets 141 in the resin matrix 142. Without applying a voltage to the liquid crystal 136, the liquid crystal molecules in the droplets 141 align in irregular directions, as shown in FIG. 1(a). In this state, a difference in refractive index exists between the liquid crystal droplets 141 and the resin 142 to scatter the incident light. By applying a voltage to the liquid crystal 136, the liquid crystal molecules are aligned in a direction, as shown in FIG. 1(b). If the refractive index of the liquid crystal component oriented in the direction is controlled preliminarily to coincide with that of the resin component, the incident light is not scattered in the liquid crystal, but transmits through it. On the other hand, the polymer network liquid crystal uses the irregularity itself of the orientation of the liquid crystal molecules for scattering the light. In the irregular orientation state, or without applying a voltage to the liquid crystal, the incident light is scattered through the liquid crystal, while when the molecules align in a direction by applying a voltage to the liquid crystal, the light transmits through the liquid crystal. The above-mentioned liquid crystal/resin composite does not need a polarizer and an analyzer for modulating the light. Therefore, it can display an image at a high luminance by twice or more when compared with a display panel with use of a twisted nematic liquid crystal.
U.S. Pat. No. 4,435,047 disclosed a prior art fabrication method of a liquid crystal panel including a liquid crystal/resin composite. A mixture liquid comprising a nematic liquid crystal and a polyvinyl alcohol (PVA) as a polymer is applied to an electrode substrate with a print technique, and it is interposed between the substrate and a counterelectrode substrate. However, it is difficult to realize a uniform thickness of liquid crystal by using the print technique, so that the scattering of brightness arises in the display image of the liquid crystal panel. Further, because the polyvinyl alcohol is a water-soluble polymer, the water resistance is bad and the liquid crystal panel becomes white and swells. Further, it need a long time to set the resin.
Japanese Patent laid open Publication 3-58021/1991 discloses a liquid crystal panel with a liquid crystal/resin composite. Two electrode substrates are arranged so as to oppose the electrodes each other, and spacers such as beads are scattered between the two substrates. The periphery of the two substrates are sealed with an epoxy sealant except an injection hole to complete an empty cell. Then, a mixture liquid comprising a liquid crystal and uncured resin is injected through the injection hole and the injection hole is sealed. Then, ultraviolet rays irradiate the mixture liquid to set the resin so as to separate the resin from the liquid crystal.
In the above-mentioned liquid crystal panel, and uniform thickness of the liquid crystal/resin composite is realized because spacers such as beads are used, and the brightness is also uniform in the display area due to the spacers. However, the beads 137 also exist in the display area as shown in FIG. 2, and this causes a problem. A liquid crystal/resin composite 136 has a thickness enough to scatter light sufficiently, while if the thickness becomes high, a higher electric voltage is needed to make the composite layer transparent. Usually, the thickness of the composite layer 136 is selected to be about 15 .mu.m. On the other hand, the size of a pixel in the display area in a liquid crystal panel has a tendency to become smaller. For example, if the display area is three inches square and includes about 300,000 or 1,000,000 pixels, a pixel occupies an area of about 100 or 50 .mu.m square. Even if the area of a pixel is small, the thickness of the liquid crystal/resin composite layer has to be as high as about 15 .mu.m. If beads 137 of about 15 .mu.m of diameter are used, the light transmits beads 137 which occupy spaces in the display area. Because the size of the beads 137 is not so small as compared with the size of a pixel such as 50 .mu.m, the light transmitting the beads 137 in the display area or the beads 137 do not modulate the light and remains as white points which are remarkable in the display area. This deteriorates the display quality especially pixels of small area.
There is another fabrication technique to form bank-like supporters on the source lines for applying an electric voltage to the pixels (Japanese Patent laid open Publication 4-84121/1992). If this technique is applied to a liquid crystal/resin composite panel, the height of the bank-like supporters have to be as high as about 15 .mu.m. However, it is difficult to form such supporters of 15 .mu.m height by using the patterning technique. Further, even if such supporters can be formed, it is difficult to inject a mixture liquid inside the supporters.